1. Field of the Invention
The present invention relates generally to electronic packaging technology and, more particularly, to a three-dimensional multi-chip stack package and a method for manufacturing such chip stack packages.
2. Description of the Related Art
With the rapid advancement of very large scale integrated circuit (VLSI) technologies, traditional packaging is becoming a performance bottleneck of the advanced microelectronics devices. This is due to the rapidly increasing of pin count and clock speed on the advanced devices. Along with the fast clock speed, power distribution of the advanced devices also is an important issue that must be addressed. Furthermore, the chip size of the advanced devices continues to grow although sub-micron processing technologies continue to reduce the minimum feature sizes on the devices. Multi-chip packaging technology is a packaging technique that has been developed to address some of the problems associated with conventional single-chip packaging.
A three-dimensional chip stack packaging has been introduced as part of the multi-chip packaging. FIG. 1 illustrates a conventional chip stack package 500 that includes two chips 511, 513 stacked on a common substrate 520. Specifically, a lower chip 511 is attached through an adhesive layer 531 on the substrate 520 so that an active surface, on which chip pads 512 are formed, faces away from the substrate (upward). An upper chip 513, which includes an active surface having chip pads 514, is attached to the active surface of the lower chip 511 using an adhesive layer 533 so that chip pads 514 face upward. Bonding wires 541, 543 are then used to form electrical connection between the chip pads 512, 514 of the chips 511, 513 and the substrate 520. An encapsulating body 550 is then formed to surround and protect the chips 511 and 513, the wires 541 and 543, and at least a portion of the top surface of the substrate 520 from the environment. A series of solder balls 560 are formed on a bottom surface of the substrate 520 to provide external electrical connections for the chips 511, 513.
Relative to single chip packaging, however, such conventional multi-chip stack packaging methods, however, tend to incur increased manufacturing time and cost associated with stacking the individual chips. In order to avoid these issues, wafer-level chip stacking has been considered to be an option for three-dimensional packaging. The ability to stack and connect multiple chips, before separating the individual chips from their parent wafers, offers several benefits over conventional chip stacking techniques including reduced manufacturing time and reduced cost
FIG. 2 illustrates a conventional wafer-level chip stack package. As illustrated in FIG. 2, at least one overlying wafer 610 and a single bottom wafer 610a, each of which may be composed of hundreds or thousands of undivided chips, are stacked together using intermediate films 630, typically a kind of anisotropic conductive film (ACF). Before stacking, each of the wafers 610, 610a is covered with a passivation layer 613 and an insulating layer 614 that protect the chip circuitry (not shown) while exposing the chip pads 612. Metal vias 617 are formed through the overlying wafers 610, starting from an upper surface of the insulating layer 614 and extending to a lower surface of the wafers. Metal traces 615 connect the chip pads 612 to a top end of the corresponding metal vias 617. The intermediate film 630 provides a connection between the bottom end of the metal vias 617 and the corresponding metal trace 615 arranged on the next lower overlying wafer 610 or the bottom wafer 610a. 
Unfortunately, wafer-level chip stacking processes as illustrated in FIG. 2 tend to suffer increased yield losses because a defective chip located on any of the included wafers 610, 610a will cause the chip-stack within which the defective chip is incorporated to fail. The failures caused by a single defective chip result in the lost of all of the properly functioning chips incorporated into the same chip stack. The number of properly functioning chips that will be lost, as well as the risk of incorporating a defective chip in a particular chip stack, increases with the number of wafers being stacked.